This invention relates generally to RF power transistors, and more particularly the invention relates to improving performance of an RF transistor having multiple cascaded transistor cells by reducing phase offsets between the cells.
A power transistor typically comprises a plurality of transistor cells or unit elements in a semiconductor die with the cells connected in parallel for amplifying an input signal. FIG. 1 is a plan view of a conventional power transistor in a semiconductor die 10 and including a plurality of cells 12. For an LDMOS transistor, each cell includes a plurality of interdigitated drain 14 and gate 16 elements with wire bonding pads 18 and 20 respectively connected to drains 14 and gate 16 through metallization 22 and 24 as shown in FIG. 2. FIG. 3 is a schematic of the LDMOS transistor cell in which the source comprises the grounded semiconductor substrate, and FIG. 4 is a section view of one LDMOS gate and drain structure in the substrate 10 for an n-channel transistor.
As shown schematically in FIG. 5, the wire bonds connected to the gate input and to the drain output have associated input inductance 28 and output inductance 30 which combine with parasitic input capacitance 32 and output capacitance 34 to modify input and output impedances. FIG. 6 is a plan view illustrating an input transmission line 40, input wire leads 42 from transmission line 40 to bonding pads 20 of cells 12, and wire leads 44 from bonding pads 18 to output transmission line 46.
For wireless phone applications, signals with short wavelengths (up to the GHz range) must be amplified. Heretofore, the multicell LDMOS transistor has performed satisfactorily for frequencies up to 1 GHz. See FIG. 7 which illustrates gate width versus CW power capability. For the 1 GHz curve 50, a linear relationship exists between gate width and power. However, 2 GHz curve 52 shows a nonlinear effect where the cascading of additional cells produces a lesser increase in power.
The present invention is directed to providing a power transistor amplifier with improved power and performance capability.